Communicating over power distribution media

ABSTRACT

A system includes a first communication module to be coupled to a first transmission medium for distributing power using a voltage waveform having a first amplitude; and a second communication module to be coupled to a second transmission medium for distributing power using a voltage waveform having a second amplitude different from the first amplitude, the second transmission medium being coupled to the first transmission medium. Each of the first and second communication modules is configured to use signals that propagate between the first and second transmission media.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/473,456, filed on May 28, 2009, which claims priority to U.S.Provisional Application Ser. No. 61/057,794, filed on May 30, 2008.

TECHNICAL FIELD

This description relates to communicating over power distribution media.

BACKGROUND

Power utility companies are deploying Smart Power Grid (or “Smart Grid”)technologies as a means of increasing reliability and efficiency ofexisting electrical power generation and distribution infrastructure.Smart Grid technology includes network infrastructure for communicatingwith “smart meters” that are located at customer premises such asresidences or other buildings for providing various monitoring andcontrol functionality. For example, the smart meter can receive DemandResponse (DR) signals originated from the power utility company, and cantransmit information such as power consumption information to the powerutility company. Smart meters can also be used to relay DR signalsreceived from a power utility company to provide Command & Control (C&C)signals within the customer premises for controlling major electricalloads during peak demand periods. For example, “Load Shedding” is atechnique that allows a public utility to control loads in order toreduce electrical demand at peak hours to avoid brown out and/or blackout conditions.

SUMMARY

In one aspect, in general, a system includes: a first communicationmodule to be coupled to a first transmission medium for distributingpower using a voltage waveform having a first amplitude; and a secondcommunication module to be coupled to a second transmission medium fordistributing power using a voltage waveform having a second amplitudedifferent from the first amplitude, the second transmission medium beingcoupled to the first transmission medium. Each of the first and secondcommunication modules is configured to use signals that propagatebetween the first and second transmission media.

Aspects can include one or more of the following features.

The second transmission medium is coupled to the first transmissionmedium by a transformer.

The first transmission medium comprises at least a first length of wire,and the second transmission medium comprises at least a second length ofwire.

The second transmission medium is coupled to the first transmissionmedium by at least a portion of the first length of wire beingdistributed in proximity to at least a portion of the second length ofwire.

The second communication module is coupled to a thermostat and isconfigured to use signals to control the thermostat.

The first communication module is coupled to a power line.

The first communication module comprises a smart meter coupled to aSmart Power Grid.

The first communication module is configured to operate in a bandwidth,wherein at least a portion of the bandwidth is above about 15 MHz.

The first communication module is configured to use Orthogonal FrequencyDivision Multiplexing (OFDM).

The first communication module is configured to use Orthogonal FrequencyDivision Multiplexing (OFDM) and binary frequency shift keying (BFSK)modulation.

The second communication module is configured to use OrthogonalFrequency Division Multiplexing (OFDM).

The second communication module is configured to use binary frequencyshift keying (BFSK) modulation.

The second communication module is configured to support data rates thatare less than about 200 kbps.

The first transmission medium comprises an Alternating Current (AC)domestic power distribution medium.

The second transmission medium comprises a Heating, Ventilation, and AirConditioning (HVAC) power distribution medium.

The first amplitude is in the range of about 100-240 Volts rms.

The second amplitude is about 24 Volts rms.

The first and second voltage waveforms have a frequency in the range ofabout 50-60 Hz.

In another aspect, in general, a method includes configuring a firstcommunication module coupled to a Heating, Ventilation, and AirConditioning (HVAC) power distribution medium to use signals thatpropagate between an Alternating Current (AC) domestic powerdistribution medium and the HVAC power distribution medium.

Aspects can include one or more of the following features.

The method further includes configuring a modulation scheme of a secondcommunication module coupled to the AC domestic power distributionmedium and configured to provide signals used by the first communicationmodule.

Configuring the modulation scheme of the second communication modulecomprises selecting at least one pair of subcarrier frequencies forFrequency Shift Keying (FSK) modulation from a plurality of subcarrierfrequencies used for Orthogonal Frequency Division (OFDM) modulation.

Selecting at least one pair of subcarrier frequencies comprisesselecting a plurality of pairs of subcarrier frequencies.

The method further includes transmitting a given signal from the secondcommunication module using FSK modulation on multiple pairs ofsubcarriers having the selected subcarrier frequencies.

The multiple pairs of subcarrier frequencies are separated from eachother by a minimum number of intervening frequencies of subcarriers thatare not used to transmit signals.

The method further includes spreading the pairs of subcarrierfrequencies from each other over the plurality of subcarrier frequenciesused for OFDM modulation to compensate for frequency selective fading onthe power distribution media.

Configuring the first communication module comprises selecting one ofthe pairs of subcarriers on which the given signal is received based ona signal quality parameter.

The signal quality parameter comprises signal-to-noise ratio.

The method further includes using the selected pair of subcarriers totransmit signals from the first communication module using FSKmodulation.

At least some of the selected pairs of subcarrier frequencies areadjacent frequencies separated by a frequency that is an inverse of asymbol time containing a whole number of periods of the sinusoidalwaveform of each of multiple mutually orthogonal subcarriers.

The symbol time is used for generating symbols within signals sentbetween the second communication module and the first communicationmodule.

The signals sent between the second communication module and the firstcommunication module use FSK modulation.

A modulation index for at least some of the selected pairs of subcarrierfrequencies is on the order of one.

A modulation index for at least some of the selected pairs of subcarrierfrequencies is between about 0.6 and about 1.5.

A modulation index for at least some of the selected pairs of subcarrierfrequencies is approximately equal to one.

The method further includes configuring a third communication modulecoupled to the AC domestic power distribution medium to use signals thatpropagate from the first communication module.

Configuring the modulation scheme of the second communication modulecomprises using Frequency Shift Keying (FSK) modulation forcommunication with the first communication module and using OrthogonalFrequency Division (OFDM) modulation for communication with the thirdcommunication module.

The method further includes configuring a third communication modulecoupled to the HVAC power distribution medium to use signals thatpropagate between the AC domestic power distribution medium and the HVACpower distribution medium.

The method further includes communicating in parallel between the firstand second communication modules and between the first and thirdcommunication modules.

Configuring the modulation scheme of the second communication modulecomprises using a first pair of subcarrier frequencies for FrequencyShift Keying (FSK) modulation for communication with the firstcommunication module and using a different second pair of subcarrierfrequencies for Frequency Shift Keying (FSK) modulation forcommunication with the third communication module.

Configuring the modulation scheme of the second communication modulecomprises using Frequency Shift Keying (FSK) modulation forcommunication with the first communication module and using OrthogonalFrequency Division (OFDM) modulation for communication over adistribution cable from an AC domestic power grid.

In another aspect, in general, a method includes communicating with aHeating, Ventilation, and Air Conditioning (HVAC) device coupled tolower voltage power distribution wiring in a premises by couplingsignals to higher voltage power distribution wiring in the premiseswithout detecting the signals before coupling the signals between thehigher voltage power distribution wiring and the lower voltage powerdistribution wiring.

In some aspects, the HVAC device comprises a thermostat.

In another aspect, in general, an apparatus comprises a communicationmodule coupled to a Heating, Ventilation, and Air Conditioning (HVAC)power distribution medium configured to use signals that propagatebetween an Alternating Current (AC) domestic power distribution mediumand the HVAC power distribution medium.

Among the many advantages of the invention (some of which may beachieved only in some of its various aspects and implementations) arethe following.

Within customer premises, the Heating, Ventilation, and Air Conditioning(HVAC) system is typically the largest electrical load. HVAC systems aretypically controlled by a thermostat. Existing “dumb” thermostats can bereplaced with “smart” thermostats that include a communication modulethat is able to receive C&C signals relayed by a smart meter outside thecustomer premises. In some cases C&C signals from a smart meter arebridged to a different format (e.g., using a wired bridge or using awireless technology such as Zigbee) for distribution of the C&C signalsto thermostats and other devices within the customer premises.

The approaches described herein enable a smart meter to relay C&Csignals to electrical loads within the customer premises directly overthe existing power lines without requiring a bridge device to convertthe C&C signals into a different format for distribution tocommunication modules at the electrical loads. Thus, the overall systemcomplexity and cost can be reduced.

A power line communication module in a smart meter can use any of avariety of power line communication protocols, such as a HomePlugprotocol (e.g., HomePlug 1.0 and HomePlugAV protocols for coupling toin-home power line networks, and HomePlug Broadband over Power Line(BPL) networks for coupling to a broadband backhaul network). Power linecommunications typically occur over a wide range of frequencies (e.g.,in some cases in the range of about 2-28 MHz, or in some cases at lowerfrequencies such as below around 500 kHz). Signals from devices that uselower frequencies (e.g., below around 500 kHz) may not be able totraverse the step-down transformer between the HVAC and AC power domainsas well as signals operating at higher frequencies (e.g., above around10-15 MHz). Therefore signals at higher frequencies may be more reliablethan signals at lower frequencies.

Equipping communication modules of electrical loads (called “C&Cclients”) with conventional power line communications capability wouldfacilitate operations above, e.g., 15 MHz, would be robust againstfrequency selective fading, and would be interoperable with similarpower line devices. Alternatively, in some cases, the high data rates(e.g. 1-200 Mbps) supported by power line protocols that use OFDM arenot necessary for some low data rate communications such as typical C&Csignals (e.g., about less than about 200 kbps, 100 kpbs, 10 kbps, or 1kbps). For example, C&C signals can include simple commands such as“ON”, “OFF”, “Reduce Power Consumption by 50%.” Thus, to further reducecost in communication modules of C&C clients to be controlled, othermodulation techniques such as Frequency Shift Keying (FSK) modulationcan be used. The communication module in the C&C client can include asimple low-cost FSK radio, and the communication module in the smartmeter can support both FSK modulation for communication with the C&Cclient and OFDM modulation for communication with the power utilitycompany and with other power line devices within the customer premises.Such a system enables high rate power line devices to communicatedirectly with low rate C&C clients.

Other aspects and advantages will be apparent from the detaileddescription, drawings, appendices and claims.

DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic diagram of a customer premises in communicationwith a power utility company.

FIG. 1B is a plot of OFDM subcarrier frequencies.

FIG. 1C is a plot of FSK channel frequencies.

FIG. 1D is a plot of an FSK frequency trajectory.

FIG. 1E is a plot of frequency selective fading.

FIG. 2 is a block diagram of an exemplary communication system.

DETAILED DESCRIPTION

There are a great many possible implementations of the invention, toomany to describe herein. Some possible implementations that arepresently preferred are described below. It cannot be emphasized toostrongly, however, that these are descriptions of implementations of theinvention, and not descriptions of the invention, which is not limitedto the detailed implementations described in this section but isdescribed in broader terms in the claims.

Referring to FIG. 1A, an HVAC system 100 in customer premises 101 iscontrolled using a control device 102A. Such control devices (e.g.,thermostats) are typically powered by a 24 Volt waveform distributedover HVAC power distribution wiring 104. For example, the HVAC powerdistribution wiring 104 may consist of a pair of wires in a bundleconsisting of several color-coded wires (e.g., blue, red, white, green,yellow, and brown). Typically, blue and red colored wires are used toprovide the 24 Volt waveform for powering HVAC devices such as “smart”thermostats that include electronics to be powered (some “dumb”thermostats include a series of mercury switches and do not requirepower). The other wires in the bundle are used to provide some form ofbinary on/off control for an HVAC system (e.g., begin heating or coolingor turning on a fan) when one or more of them are shorted together(e.g., by a switch that forms a conductive path between them).

The 24 Volt waveform is derived from a higher voltage waveform of the ACdomestic power distribution wiring 106 (at 120 Volts or 240 Volts, forexample) through a step-down transformer 108. While in the past it hasbeen assumed that existing thermostats would be replaced by “smart”thermostats that include wireless communications modules to receive C&Csignals, tests have been performed to determine that typical insertionlosses at typical power line communication frequencies (e.g., 20-30 MHz)from the AC domestic power distribution wiring 106 to the HVAC powerdistribution wiring 104 through a typical transformer are around 30 dB.In addition, test measurements in actual homes indicate that the actualinsertion loss can be much lower, for example, about 15 dB at somepoints in a 2-28 MHz band. Without intending to be bound by theory, onepossible explanation for why insertion losses can be lower in some homesis that increased coupling occurs between the AC domestic powerdistribution wires and the 24 Volt HVAC power distribution wires as theyrun along common paths as they are distributed around the home.

These levels of losses are surmountable using the techniques describedherein to enable a smart meter to control HVAC devices (e.g.,thermostats) directly over power lines without bridging C&C signalsbetween the AC domestic power distribution medium and the HVAC powerdistribution medium (e.g., without using circuitry to detect andretransmit C&C signals or otherwise bypass the transformer).

Referring again to FIG. 1A, a power utility company head end 110 cancommunicate with a smart meter 112 or other Customer Premises Equipment(CPE) that serves as a gateway modem for receiving C&C signals. Thecommunication network 114 between the head end 110 and the smart meter112 is referred to as a “backhaul” network. C&C signals arrive over thebackhaul network and are interpreted by the smart meter 112 tocommunicate with individual C&C clients within the customer premisesincluding the HVAC control device 102A, other HVAC devices 102B-102C,and other devices 102D -102F connected to the AC domestic powerdistribution wiring 106 in the premises 101.

In many homes, detection of the C&C signals on one side of thetransformer 108 and retransmission on the other side of the transformer108 would require installation of a bridge device that detects andretransmits signals, e.g., using a different medium, a differentprotocol, or both. Eliminating the requirement for such a bridge devicecould facilitate conversion to a Smart Power Grid system (e.g., bysaving time and cost of installing the bridge device). Instead ofrequiring a bridge device to be installed, by properly configuring themodulation scheme used by the smart meter 112, the C&C signals can becoupled to the AC domestic power distribution wires without detectingthe signals before they couple to the HVAC power distribution wires (bydirect uninterrupted propagation across the transformer from one mediumto the other) and the C&C signals can be detected and used by an HVACC&C client. Alternatively, in some homes, a passive bypass device may beinstalled to allow communication signals to bypass the transformer 108without propagating through it. However, use of such a bypass device mayrequire an installer to physically locate the transformer to install thebypass device. Eliminating the requirement for such a bypass devicecould also save time and cost.

There are a variety of waveforms that can be employed, such as varioustypes of OFDM modulated waveforms, including HomePlug ROBO waveforms(e.g., as described in U.S. Pat. No. 6,278,685, incorporated herein byreference), to facilitate reliable bi-directional communications betweena smart meter and a C&C client, including HVAC C&C clients.

In some implementations, single carrier direct sequence spread spectrum(DSSS) modulation can be employed. Regulatory requirements limitradiated emissions from power lines carrying communication signals. Inthe USA, this limit is −41 dBm/MHz. Use of a modulation scheme such asDSSS will allow the low date rate narrow band C&C signal to be spreadover an arbitrary bandwidth, thereby allowing a much higher totaltransmitted power than would otherwise be possible. In some cases, useof DSSS helps mitigate the effects of multipath interference.

The potential drawback to power line C&C systems operating at lowerfrequencies is that signals at lower frequencies to not couple acrosselectrical phases in household wiring and they do not propagate onto the24 Volt HVAC wiring system as well. Households are normally providedwith 240 Volt AC electrical power, which is split at a distribution boxinto two waveforms having different phases operating at 120 Volt AC. Insome cases, to increase reliability, power line communications devicescommunicate across both phases. As described above, C&C signals canreach a thermostat to control an HVAC system requires by propagatingdirectly, without bridging from the AC domestic power distributionmedium, through the step-down transformer. Experience has shown thathigher frequencies (above 10-15 MHz) can be more reliable for both crossphase and AC domestic power to provide HVAC control communications.

In some implementations, the HVAC C&C clients can include communicationmodules that include 2-level or “binary” FSK (BFSK) radios. These radioswill support low data rates (e.g., 24-120 kbps) which are adequate forthe C&C application. In addition, FSK radios can be implementedextremely inexpensively compared to more flexible radios used in thesmart meter, which also support OFDM modulation. Advanced signalprocessing capabilities already installed in power line communicationdevices (e.g., HomePlug devices) can be configured (e.g., usingsoftware, firmware, and/or hardware) to provide support for both OFDMmodulation and FSK modulation. The end result is little or no marginalcost increase in the power line device to be used in a smart meter.

By properly configuring the BFSK communication modules in the C&Cclients and the modulation scheme used by the OFDM-based communicationmodule in the smart meter, C&C communication can occur directly at thephysical (PHY) layer without higher layer conversion of data modulationor encoding. For example, this can be accomplished by using a selectedsubset of existing OFDM subcarriers as mark tones (used to encode abinary value of “1” at a first frequency) and space tones (used toencode a binary value of “0” at a second frequency). FIG. 1B shows atypical set of OFDM subcarriers, which can each be modulated inamplitude and phase. Multiple pairs of subcarriers can be selected andused for multiple FSK channels to provide diversity for overcomingfrequency dependent fading. By selecting substantially uniformly spacedadjacent subcarrier pairs, as shown in FIG. 1C, and leaving theintervening subcarriers off, channel spacing is relatively large, whichminimizes the possibility of Adjacent Channel Interference (ACI). At thesame time, there are still enough channels to ensure that low cost C&Cdevices on the client side could locate a narrowband channel that is notaffected by frequency selective fading.

The FSK C&C clients would be frequency agile, with the ability to changeFSK channels quickly, in order to overcome frequency selective fading.During modulation on a given FSK channel, the frequency is changedbetween the mark and space frequencies. FIG. 1D shows an example of afrequency trajectory for FSK modulation. In some implementations, thefrequency is smoothly transitioned between a mark frequency f2 and aspace frequency f1 over a symbol time period T. FIG. 1E shows an exampleof frequency selective fading over a range of frequencies from 500 kHzto 30 MHz for a typical power line environment.

FSK radio performance is dependent on modulation index (frequencyseparation between mark & space frequencies divided by symbol rate).Subcarrier spacing in OFDM radios is intimately related to the symboltime (as described in more detail below). In order to achieve subcarrierorthogonality in the frequency domain, the subcarriers are spaced at afrequency that is the inverse of the symbol time. This means that for anFSK C&C client using adjacent subcarriers for mark & space tones, themodulation index is about 1.0 (or somewhat smaller or larger than 1.0 ifdifferent symbol time durations are used for OFDM and FSK signals),which results in very robust signaling. The FSK modulation parameters(e.g., mark & space frequency separation and symbol period) can beselected so that the existing parameters used for OFDM modulation (e.g.,DFT parameters that determine the number of samples in a symbol, thesymbol time, etc.) can be preserved.

By creating a relatively large number (e.g., 100) of substantiallyuniformly spaced FSK channels (each FSK channel consists of two adjacentsubcarriers) and employing frequency agility, the FSK C&C client can bereliable in the presence of frequency selective fading. For example,during an acquisition phase, or at regular intervals, the smart metercan broadcast on all (or a predetermined subset of) FSK channels. TheFSK C&C client can select the FSK channel with the best signalingcharacteristics (e.g., signal-to-noise ratio). This selected FSK channelmay last for a relatively long time (e.g., many seconds) before itbecomes necessary to select a new FSK channel (e.g., due to fadingand/or increased noise). Further, multiple C&C clients are able tocommunicate in parallel with the smart meter (selecting different FSKchannels) to reduce the impact of C&C communications on overall networkthroughput. Finally, by leaving the majority of subcarriers unused andilluminating substantially uniformly spaced subcarrier pairs, AdjacentChannel Interference ACI can be reduced when multiple clients arecommunicating with the smart meter via OFDMA techniques.

Any of a variety of communication system architectures can be used toimplement the OFDM communication module in the smart meter that can beconfigured, as described above, to communicate using FSK modulation inaddition to OFDM modulation. An application running on the smart meterprovides and receives data to and from a network interface module insegments. A “MAC Service Data Unit” (MSDU) is a segment of informationreceived by the MAC layer. The MAC layer can process the received MSDUsand prepares them to generate “MAC protocol data units” (MPDUs). An MPDUis a segment of information including a header (e.g., with managementand overhead information) and payload fields that the MAC layer hasasked the PHY layer to transport. An MPDU can have any of a variety offormats based on the type of data being transmitted. A “PHY ProtocolData Unit (PPDU)” refers to the modulated signal waveform representingan MPDU that is transmitted over the power line by the physical layer.

Apart from generating MPDUs from MSDUs, the MAC layer can provideseveral functions including channel access control, providing therequired QoS for the MSDUs, retransmission of corrupt information,routing and repeating. Channel access control enables stations to sharethe powerline medium. Several types of channel access control mechanismslike carrier sense multiple access with collision avoidance (CSMA/CA),centralized Time Division Multiple Access (TDMA), distributed TDMA,token based channel access, etc., can be used by the MAC. Similarly, avariety of retransmission mechanism can also be used. The Physical layer(PHY) can also use a variety of techniques to enable reliable andefficient transmission over the transmission medium (power line, coax,twisted pair etc). Forward error correction (FEC) code line Viterbicodes, Reed-Solomon codes, concatenated code, turbo codes, low densityparity check code, etc., can be employed by the PHY to overcome errors.

In OFDM modulation, data are transmitted in the form of OFDM “symbols.”Each symbol has a predetermined time duration or symbol time T_(s). Eachsymbol is generated from a superposition of N sinusoidal carrierwaveforms that are orthogonal to each other and form the OFDM carriers.Each carrier has a peak frequency f_(i) and a phase Φ_(i), measured fromthe beginning of the symbol. For each of these mutually orthogonalcarriers, a whole number of periods of the sinusoidal waveform iscontained within the symbol time T_(s). Equivalently, each carrierfrequency is an integral multiple of a frequency interval Δf=1/T_(s).The phases Φ_(i) and amplitudes A_(i) of the carrier waveforms can beindependently selected (according to an appropriate modulation scheme)without affecting the orthogonality of the resulting modulatedwaveforms. The carriers occupy a frequency range between frequenciesf_(i) and f_(N) referred to as the OFDM bandwidth.

Referring to FIG. 2, a communication system 200 includes a transmitter202 for transmitting a signal (e.g., a sequence of OFDM symbols) over acommunication medium 204 to a receiver 206. The transmitter 202 andreceiver 206 can both be incorporated into a network interface module ateach station. The communication medium 204 can represent a path from onedevice to another over the power line network.

At the transmitter 202, modules implementing the PHY layer receive anMPDU from the MAC layer. The MPDU is sent to an encoder module 220 toperform processing such as scrambling, error correction coding andinterleaving.

The encoded data is fed into a mapping module 222 that takes groups ofdata bits (e.g., 1, 2, 3, 4, 6, 8, or 10 bits), depending on theconstellation used for the current symbol (e.g., a BPSK, QPSK, 8-QAM,16-QAM constellation), and maps the data value represented by those bitsonto the corresponding amplitudes of in-phase (I) and quadrature-phase(Q) components of a carrier waveform of the current symbol. This resultsin each data value being associated with a corresponding complex numberC_(i)=A_(i) exp(jΦ_(i)) whose real part corresponds to the I componentand whose imaginary part corresponds to the Q component of a carrierwith peak frequency_(i). Alternatively, any appropriate modulationscheme that associates data values to modulated carrier waveforms can beused.

The mapping module 222 also determines which of the carrier frequenciesf_(N) within the OFDM bandwidth are used by the system 200 to transmitinformation. For example, some carriers that are experiencing fades canbe avoided, and no information is transmitted on those carriers.Instead, the mapping module 222 uses coherent BPSK modulated with abinary value from the Pseudo Noise (PN) sequence for that carrier. Forsome carriers (e.g., a carrier i=10) that correspond to restricted bands(e.g., an amateur radio band) on a medium 204 that may radiate power noenergy is transmitted on those carriers (e.g., A₁₀=0). The mappingmodule 222 also determines the type of modulation to be used on each ofthe carriers (or “tones”) according to a “tone map.” The tone map can bea default tone map, or a customized tone map determined by the receivingstation, as described in more detail below.

An inverse discrete Fourier transform (IDFT) module 224 performs themodulation of the resulting set of N complex numbers (some of which maybe zero for unused carriers) determined by the mapping module 222 onto Northogonal carrier waveforms having peak frequencies f₁, . . . f_(N).The modulated carriers are combined by IDFT module 224 to form adiscrete time symbol waveform S(n) (for a sampling rate f_(R)), whichcan be written as

$\begin{matrix}{{S(n)} = {\sum\limits_{i = 1}^{N}{A_{i}{\exp \left\lbrack {j\left( {{2\pi \; \; {n/N}} + \Phi_{i}} \right)} \right\rbrack}}}} & {{Eq}.\mspace{14mu} (1)}\end{matrix}$

where the time index n goes from 1 to N, Ai is the amplitude and Φ_(i)is the phase of the carrier with peak frequency f_(i)=(i/N)f_(R), andj=√−1. In some implementations, the discrete Fourier transformcorresponds to a fast Fourier transform (FFT) in which N is a power of2.

A post-processing module 226 combines a sequence of consecutive(potentially overlapping) symbols into a “symbol set” that can betransmitted as a continuous block over the communication medium 204. Thepost-processing module 226 prepends a preamble to the symbol set thatcan be used for automatic gain control (AGC) and symbol timingsynchronization. To mitigate intersymbol and intercarrier interference(e.g., due to imperfections in the system 200 and/or the communicationmedium 204) the post-processing module 226 can extend each symbol with acyclic prefix that is a copy of the last part of the symbol (also calleda “guard interval”). The post-processing module 226 can also performother functions such as applying a pulse shaping window to subsets ofsymbols within the symbol set (e.g., using a raised cosine window orother type of pulse shaping window) and overlapping the symbol subsets.

An Analog Front End (AFE) module 228 couples an analog signal containinga continuous-time (e.g., low-pass filtered) version of the symbol set tothe communication medium 204. The effect of the transmission of thecontinuous-time version of the waveform S(t) over the communicationmedium 204 can be represented by convolution with a function g(τ;t)representing an impulse response of transmission over the communicationmedium. The communication medium 204 may add noise n(t), which may berandom noise and/or narrowband noise emitted by a jammer.

At the receiver 206, modules implementing the PHY layer receive a signalfrom the communication medium 204 and generate an MPDU for the MAClayer. An AFE module 230 operates in conjunction with an Automatic GainControl (AGC) module 232 and a time synchronization module 234 toprovide sampled signal data and timing information to a discrete Fouriertransform (DFT) module 236.

After removing the cyclic prefix, the receiver 206 feeds the sampleddiscrete-time symbols into DFT module 236 to extract the sequence of Ncomplex numbers representing the encoded data values (by performing anN-point DFT). Demodulator/Decoder module 238 maps the complex numbersonto the corresponding bit sequences and performs the appropriatedecoding of the bits (including de-interleaving and descrambling).

Any of the modules of the communication system 200 including modules inthe transmitter 202 or receiver 206 can be implemented in hardware,software, or a combination of hardware and software.

In some implementations, synchronization techniques can be used, forexample, to synchronize communication to a power line cycle, asdescribed in more detail in U.S. application Ser. No. 11/337,946,incorporated herein by reference. In some implementations, the powerline medium can also be coupled to other media such as coaxial cable, asdescribed, for example, in U.S. application Ser. No. 11/200,910,incorporated herein by reference.

Many other implementations of the invention other than those describedabove are within the invention, which is defined by the followingclaims.

What is claimed is:
 1. A system comprising: a network interface; and afirst communication module configured to be coupled with a firsttransmission medium for distributing power using a voltage waveformhaving a first amplitude in a network that also comprises a secondtransmission medium for distributing power using a voltage waveformhaving a second amplitude different from the first amplitude, the firsttransmission medium being coupled with the second transmission medium;the first communication module further configured to: utilize signalsthat propagate between the first and second transmission media forcommunication with at least a second communication module coupled withthe second transmission medium; and implement a modulation scheme thatenables direct uninterrupted propagation of the signals from the firsttransmission medium to the second transmission medium.
 2. The system ofclaim 1, wherein the first transmission medium is coupled with thesecond transmission medium by a transformer.
 3. The system of claim 1,wherein the first communication module is configured to implement amodulation scheme that enables direct uninterrupted propagation of thesignals from the first transmission medium to the second communicationmodule coupled with the second transmission medium via a transformer. 4.The system of claim 3, wherein the second communication module iscoupled with a thermostat and is configured to use signals to controlthe thermostat.
 5. The system of claim 1, wherein the firstcommunication module is coupled to a power line.
 6. The system of claim5, wherein the first communication module comprises a smart metercoupled to a Smart Power Grid.
 7. The system of claim 6, wherein thefirst transmission medium comprises an Alternating Current (AC) domesticpower distribution medium.
 8. The system of claim 1, wherein the firstamplitude is in a range of 100-240 Volts rms, and the second amplitudeis 24 Volts rms.
 9. The system of claim 1, wherein the first and secondvoltage waveforms have a frequency in the range of about 50-60 Hz. 10.The system of claim 1, wherein the first communication module configuredto implement a modulation scheme that enables direct uninterruptedpropagation of the signals from the first transmission medium to thesecond transmission medium comprises the first communication moduleconfigured to select at least one pair of subcarrier frequencies forFrequency Shift Keying (FSK) modulation from a plurality of subcarrierfrequencies used for Orthogonal Frequency Division (OFDM) modulation.11. The system of claim 10, wherein the first communication moduleconfigured to select at least one pair of subcarrier frequenciescomprises the first communication module configured to select aplurality of pairs of subcarrier frequencies.
 12. The system of claim10, wherein the first communication module is further configured totransmit a given signal using FSK modulation on multiple pairs ofsubcarriers having the selected subcarrier frequencies.
 13. The systemof claim 12, wherein the multiple pairs of subcarrier frequencies areseparated from each other by a minimum number of intervening frequenciesof subcarriers that are not used to transmit signals.
 14. The system ofclaim 12, wherein the first communication module configured to select atleast one pair of subcarrier frequencies comprises the firstcommunication module configured to select a plurality of pairs ofsubcarrier frequencies, and wherein the first communication module isfurther configured to spread the pairs of subcarrier frequencies fromeach other over the plurality of subcarrier frequencies used for OFDMmodulation to compensate for frequency selective fading on the powerdistribution media.
 15. The system of claim 10, wherein at least some ofthe selected pairs of subcarrier frequencies are adjacent frequenciesseparated by a frequency that is an inverse of a symbol time containinga whole number of periods of the sinusoidal waveform of each of multiplemutually orthogonal subcarriers.
 16. The system of claim 15, wherein thesymbol time is used for generating symbols within signals sent betweenthe first communication module and the second communication module. 17.The system of claim 16, wherein the signals sent between the firstcommunication module and the second communication module use FSKmodulation.
 18. The system of claim 10, wherein a modulation index forat least some of the selected pairs of subcarrier frequencies is on theorder of one.
 19. The system of claim 18, wherein a modulation index forat least some of the selected pairs of subcarrier frequencies is between0.6 and about 1.5.
 20. The system of claim 10, wherein the firstcommunication module configured to implement a modulation scheme furthercomprises the first communication module configured to use FSKmodulation for communication with the second communication module anduse OFDM modulation for communication with a third communication modulecoupled with the first transmission medium.
 21. The system of claim 10,wherein the first communication module configured to implement amodulation scheme further comprises the first communication moduleconfigured to use a first pair of subcarrier frequencies for FSKmodulation for communication with the second communication module anduse a different second pair of subcarrier frequencies for FSK modulationfor communication with a third communication module coupled with thesecond transmission medium.
 22. The system of claim 10, wherein thefirst communication module configured to implement a modulation schemefurther comprises the first communication module configured to use FSKmodulation for communication with the second communication module anduse OFDM modulation for communication over a distribution cable from anAC domestic power grid.
 23. A method comprising: configuring a firstcommunication module coupled to a first transmission medium to usesignals that propagate between the first transmission medium and asecond transmission medium for communication with at least a secondcommunication module coupled with the second transmission medium, thefirst transmission medium for distributing power using a voltagewaveform having a first amplitude and the second transmission medium fordistributing power using a voltage waveform having a second amplitudedifferent from the first amplitude, the first transmission medium beingcoupled with the second transmission medium; and implementing amodulation scheme at the first communication module that enables directuninterrupted propagation of the signals from the first transmissionmedium to the second transmission medium.
 24. The method of claim 23,wherein said implementing a modulation scheme at the first communicationmodule comprises implementing a modulation scheme that enables directuninterrupted propagation of the signals from the first transmissionmedium to the second communication module coupled with the secondtransmission medium via a transformer.
 25. The method of claim 23,wherein said implementing a modulation scheme at the first communicationmodule that enables direct uninterrupted propagation of the signals fromthe first transmission medium to the second transmission mediumcomprises selecting at least one pair of subcarrier frequencies forFrequency Shift Keying (FSK) modulation from a plurality of subcarrierfrequencies used for Orthogonal Frequency Division (OFDM) modulation.26. The method of claim 25, wherein said selecting at least one pair ofsubcarrier frequencies comprises selecting a plurality of pairs ofsubcarrier frequencies.
 27. The method of claim 25, further comprisingtransmitting a given signal from the first communication module usingFSK modulation on multiple pairs of subcarriers having the selectedsubcarrier frequencies.
 28. The method of claim 27, wherein the multiplepairs of subcarrier frequencies are separated from each other by aminimum number of intervening frequencies of subcarriers that are notused to transmit signals.
 29. The method of claim 27, wherein saidselecting at least one pair of subcarrier frequencies comprisesselecting a plurality of pairs of subcarrier frequencies, and whereinthe method further comprises spreading the pairs of subcarrierfrequencies from each other over the plurality of subcarrier frequenciesused for OFDM modulation to compensate for frequency selective fading onthe power distribution media.
 30. The method of claim 25, wherein atleast some of the selected pairs of subcarrier frequencies are adjacentfrequencies separated by a frequency that is an inverse of a symbol timecontaining a whole number of periods of the sinusoidal waveform of eachof multiple mutually orthogonal subcarriers.
 31. The method of claim 30,wherein the symbol time is used for generating symbols within signalssent between the first communication module and the second communicationmodule.
 32. The method of claim 31, wherein the signals sent between thefirst communication module and the second communication module use FSKmodulation.
 33. The method of claim 25, wherein said implementing amodulation scheme at the first communication module comprises using FSKmodulation for communication with the second communication module andusing OFDM modulation for communication with a third communicationmodule coupled with the first transmission medium.
 34. The method ofclaim 25, wherein said implementing a modulation scheme at the firstcommunication module comprises using a first pair of subcarrierfrequencies for FSK modulation for communication with the secondcommunication module and using a different second pair of subcarrierfrequencies for FSK modulation for communication with a thirdcommunication module coupled with the second transmission medium. 35.The method of claim 25, wherein said implementing a modulation scheme atthe first communication module comprises using FSK modulation forcommunication with the second communication module and using OFDMmodulation for communication over a distribution cable from an ACdomestic power grid.
 36. A system comprising: a first communicationmodule configured to be coupled to a first transmission medium fordistributing power using a voltage waveform having a first amplitude;and a second communication module configured to be coupled to a secondtransmission medium for distributing power using a voltage waveformhaving a second amplitude different from the first amplitude, the secondtransmission medium being coupled to the first transmission medium;wherein each of the first and second communication modules is configuredto use signals that propagate between the first and second transmissionmedia, wherein the first communication module is configured to implementa modulation scheme that enables direct uninterrupted propagation of thesignals from the first transmission medium to the second transmissionmedium.
 37. The system of claim 36, wherein the first communicationmodule is configured to implement a modulation scheme that enablesdirect uninterrupted propagation of the signals from the firsttransmission medium to the second communication module coupled with thesecond transmission medium via a transformer.
 38. The system of claim37, wherein the second communication module is coupled with a thermostatand is configured to use signals to control the thermostat.
 39. Thesystem of claim 36, wherein the first transmission medium comprises anAlternating Current (AC) domestic power distribution medium.
 40. Thesystem of claim 36, wherein the first communication module configured toimplement a modulation scheme that enables direct uninterruptedpropagation of the signals from the first transmission medium to thesecond transmission medium comprises the first communication moduleconfigured to select at least one pair of subcarrier frequencies forFrequency Shift Keying (FSK) modulation from a plurality of subcarrierfrequencies used for Orthogonal Frequency Division (OFDM) modulation.41. The system of claim 40, wherein the first communication moduleconfigured to select at least one pair of subcarrier frequenciescomprises the first communication module configured to select aplurality of pairs of subcarrier frequencies.
 42. The system of claim40, wherein the first communication module is further configured totransmit a given signal using FSK modulation on multiple pairs ofsubcarriers having the selected subcarrier frequencies.
 43. The systemof claim 42, wherein the multiple pairs of subcarrier frequencies areseparated from each other by a minimum number of intervening frequenciesof subcarriers that are not used to transmit signals.
 44. The system ofclaim 42, wherein the first communication module configured to select atleast one pair of subcarrier frequencies comprises the firstcommunication module configured to select a plurality of pairs ofsubcarrier frequencies, and wherein the first communication module isfurther configured to spread the pairs of subcarrier frequencies fromeach other over the plurality of subcarrier frequencies used for OFDMmodulation to compensate for frequency selective fading on the powerdistribution media.
 45. The system of claim 40, wherein at least some ofthe selected pairs of subcarrier frequencies are adjacent frequenciesseparated by a frequency that is an inverse of a symbol time containinga whole number of periods of the sinusoidal waveform of each of multiplemutually orthogonal subcarriers.
 46. The system of claim 40, wherein thefirst communication module configured to implement a modulation schemefurther comprises the first communication module configured to use FSKmodulation for communication with the second communication module anduse OFDM modulation for communication with a third communication modulecoupled with the first transmission medium.
 47. The system of claim 40,wherein the first communication module configured to implement amodulation scheme further comprises the first communication moduleconfigured to use a first pair of subcarrier frequencies for FSKmodulation for communication with the second communication module anduse a different second pair of subcarrier frequencies for FSK modulationfor communication with a third communication module coupled with thesecond transmission medium.
 48. The system of claim 40, furthercomprising a third communication module coupled to the secondtransmission medium to use signals that propagate between the firsttransmission medium and the second transmission medium.
 49. The systemof claim 48, further comprising communicating in parallel between thefirst and second communication modules and between the first and thirdcommunication modules.
 50. The system of claim 48, wherein the firstcommunication module configured to implement a modulation scheme furthercomprises the first communication module configured to use a first pairof subcarrier frequencies for Frequency Shift Keying (FSK) modulationfor communication with the first communication module and using adifferent second pair of subcarrier frequencies for Frequency ShiftKeying (FSK) modulation for communication with the third communicationmodule.
 51. The system of claim 40, wherein the first communicationmodule configured to implement a modulation scheme further comprises thefirst communication module configured to use FSK modulation forcommunication with the second communication module and use OFDMmodulation for communication over a distribution cable from an ACdomestic power grid.